I was told at my 20 week scan with DS1 that he was well above 'normal' 20 week size and he would be well over 10lb by 40 weeks. In the end he was 6lb 15oz at 38 weeks. I did have PE though, which probably reduced his growth towards the end. DS2 was big at his 20 weeks scan too, but was only 7lb 8oz at 40 weeks.
I wouldn't worry at this point. My ob told me after I was told I would be having a giant baby that he hears that all the time and it is rarely the case. The estimates are based on length of bones at 20 weeks, so could just be saying you will have a really tall baby, not necessarily fat or with a giant head (the bit you really need to worry about ).